In wireless or mobile communication systems, e.g., TDMA or CDMA based systems, multi carrier transmission schemes are used in which signals are transmitted using multiple carriers with different frequencies. This may serve to mitigate the effect of frequency-selective fading that appears for a case of high data rate transmission. However, if such multi carrier signals are passed through non-linear elements or stages of devices, distortions due to intermodulation between the two carrier signals occur. These distortions are caused by harmonic frequency components (intermodulation frequencies) generated by the non-linear characteristic of the element or stage. In particular, intermodulation distortion (IMD) products consist of higher order harmonics, e.g., IMD 3 (Intermodulation distortion third order) products caused by second harmonic components (having twice the value of the fundamental carrier frequency) and IMD 5 (intermodulation distortion fifth order) products caused by second and third harmonic components (i.e., having twice and triple the value of the fundamental carrier frequency).
FIG. 1 shows a frequency spectrum at the output of a non-linear stage such as a power amplifier of the GSM system, wherein a two-tone signal consisting of two carrier frequencies fo1 and fo2 is amplified by respective amplifiers. As can be gathered from FIG. 1, the intermodulation distortions due to the non-linear characteristics of the amplifier lead to additional harmonic components at the frequencies fo1−Δf and fo2+Δf (IMD 3 products), and at the frequencies fo1−2Δf and fo2+2Δf (IMD 5 products), wherein Δf denotes the frequency offset between the two carrier frequencies.
Therefore, a linearisation of the characteristics of such non-linear elements or stages is required in order to suppress the undesired IMD products (especially the major IMD 3products which mainly result from a third order non-linearity of the characteristics). Such a linearisation may be achieved by linearising the characteristic curve of the respective non-linear element to thereby reduce the third or fifth order intermodulation products.
Furthermore, it has been suggested to suppress the IMD products by sub-banding the operating bandwidth, such that the carrier frequencies are individually processed. However, in this case, a full bandwidth operation is not possible. Moreover, a phase coherency is required, wherein an extra carrier signal, typically the carrier of the Broadcast Control CHannel (BCCH) of the GSM system was required to achieve phase synchronization.